It is known that hot-melt adhesives may be used to join woven materials to nonwoven materials, such as the fibrous material of a disposable diaper lining to its smooth plastic casing. Typically, hot-melt adhesive is applied to a plastic diaper casing in a series of parallel continuous strips by a high pressure applicator.
While the application of hot-melt in continuous strips provides adequate bonding strength for most uses, such application has certain problems. First, the continuous strips represent a considerable use of adhesive material. And since a significant portion of the adhesive does not actually contact either the woven material or the nonwoven material, but instead solidifies within the interior of the strips, much of the adhesive does little to add to bonding strength. Of course, additional strips will increase the strength of the bond, but the additional strips will require an even greater use of adhesive.
Furthermore, continuous strips form channels and dams which prevent the free passage of liquid. Again using a disposable diaper as an example, the joinder of the polypropylene to the diaper lining along a strip hinders the side-to-side flow of liquid and tends to channel liquid forwardly or, more commonly, rearwardly out of the diaper. The problem is compounded because the mass of the adhesive maintains the temperature and will cause some plastic materials, such as polypropylene, to melt somewhat during application. Pucker lines are obvious on most disposable diapers and will increase the chances of liquid channeling, while simultaneously decreasing the bonding strength between the two materials.
One solution to some of the problems is to produce a series of regular, linear, short dots and dashes instead of a continuous strip. In U.S. Pat. No. 3,348,520, Lockwood discloses an apparatus which produces linear dots and extended dashes of hot-melt by opening and closing valves in nozzles at a high cycling rate. Valves in the dispensing head are responsive to the alternating high pressure stroke and suction stroke of a pump. The valve in each nozzle also ensures clean sharp closure of the nozzles, thereby preventing any tendency towards dripping. However, while the linear dots and extending dashes do reduce the amount of adhesive needed per application, puckering of the diaper remains a problem since the mass of adhesive at a particular locality of application remains the same. Additionally, the use of high pressure to force hot-melt through nozzle orifices presents an occupational risk and requires expensive hoses and fittings, as a rupture in the equipment could spray hot material in any direction. Moreover, to prevent excessive adhesive flow during high pressure applications, nozzle orifices must be dimensionally small, thereby leading to maintenance problems since a reduction in the size of nozzle orifices often results in a greater frequency of plugged nozzles.
It is an object of the present invention to provide a method for joining a woven material to a nonwoven material, using hot-melt adhesives, which operates at low pressures and which results in a considerable saving of the amount of hot-melt adhesive used. It is a further object to provide such a method which produces a product which is without pucker lines and which has considerably more strength of bond.